Pigment-size effect on the physico-chemical behavior of azurite-tempera dosimeters upon natural and accelerated photo aging

Effect of a SO2 Rich Atmosphere on Tempera Paint Mock-Ups. Part 2: Accelerated Aging of Azurite- and Malachite-Based Paints

“…In this way, grains of classified pigments surrounded by a protein coating were obtained [19]. When analyzing these same commercial pigments, Cardell et al [21] found that the egg yolk content added to AZ-EC, AZ-C, AZ-M, and AZ-EF due to the MP process ranged from 2 wt% to 3.5 wt%, whereas the egg yolk content in AZ-ST was significantly higher (10.5 wt%).…”

“…The tempera mock-ups were prepared by mixing one of the pigments and one of the binders according to Old Master recipes [38]. The procedure for the egg yolk-based paints can be consulted in [39] and that for the rabbit glue-based paints in [21]. The pigment-binder mixture (i.e., the tempera paint) was applied onto glass slides (ca.…”

“…As tempera paints are made by mixing inorganic pigments with a proteinaceous binder (e.g., egg yolk or rabbit glue) [18], the damage caused to them by atmospheric gases such as SO 2 must be studied from different perspectives, focusing on the changes induced by the pigments, those related with the binder and those produced by binder-pigment interaction. The literature reports that the appearance and surface finish of fresh unaltered tempera paints varied depending on the type of pigment used, the pigment particle size and the type and amount of binder present in the paints, as all these factors influenced the color of the paint and the roughness of its surface [19][20][21][22]. Other recent research has reported that the impurities found in the historic mineral pigments used in tempera paints might significantly alter their optical, chemical, and physical properties [22][23][24].…”

“…Other recent research has reported that the impurities found in the historic mineral pigments used in tempera paints might significantly alter their optical, chemical, and physical properties [22][23][24]. Although some research has been conducted on aged tempera paint mock-ups mainly exposed to artificial UV light and outdoor atmosphere, including direct sunlight radiation, very little work has been done on the effects of impurities and pigment grain size on their weathering processes [21,25,26]. Herrera et al [10] examined accelerated lime-based paint mock-ups and reported the importance of the mineralogical composition of the pigments (including impurities) and the specific surface area of the paint related to the binder.…”

“…In aqueous systems, CO 3 2− and HCO 3 − activities (and Cu 2+ activity to a lesser extent) define the stability of both azurite and malachite minerals [32]. In the field of tempera paint conservation, it has been proved that exposure to UV light can transform azurite into malachite as detected via grains with an intermediate chemical composition between both phases [21]. Also, the exposure of historical azurite tempera paints to chloride-rich water (like those of rivers or sea floods) can lead to precipitation of green paratacamite ((Cu 2+ ) 3 (Cu,Zn)(OH) 6 Cl 2 ) [33].…”